1. M. P. S. Eastham, ?On a limit-point method of Hartman,? Bull. London Math. Soc.,4, 340?344 (1972).
2. T. T. Read, ?A limit-point criterion for expressions with intermittently positive coefficients,? J. London Math. Soc.,15, 271?276 (1977).
3. Yu. B. Orochko, ?Sufficient conditions for self-adjointness of the Sturm-Liouville operator,? Mat. Zametki,15, No. 2, 271?280 (1974).
4. W. N. Everitt, ?On the deficiency index problem for ordinary differential operators, 1910?1976,? in: Differential Equations, Proceedings of the Uppsala 1977 International Conference on Differential Equations, Uppsala (1977), pp. 62?81.
5. F. V. Atkinson, ?Limit-n criteria of integral type,? Proc. R. Soc. Edinburgh,73, No. 11, 167?198 (1975).